Wireless receiving apparatus



Dec. 26, 1933. J, ROBINSON 1,941,155

WIRELESS RECEIVING APPARATUS Filed March 3. 1931 siiiywe o Faced/er v 2% yi jmfw Patented Dec. 26, 1 933 I 1,941,155 I WIRELESS RECEIVING APPARATUS James Robinson, London, England, assignor, by mesne assignments, to British Radiostat. Cor: poration, Limited, a corporation of Canada Application March 3, 1931, Serial No. 519,810, and

in Great Britain March 6, 1930 Claims. '(01. 250-) This invention is for improvements in or relating to wireless an dlilre receiving apparatus and is concerned with receivers of the type having very high selectivity and/or low damping and 5 employing for this purpose a resonant device such as a piezo-electric crystal, or a tuning fork;

or a thermionic valve system with retroaction or a plurality of tuned circuits in cascade or other filter systems. Examplesof such highly selective receivers are described in my prior United States Patents ,Nos, 1,821,032 and 1,821,033, granted September 1', 1931.

With. such receivers it is possible to. separate and receive independently any of a plurality of missible for satisfactory reception with normal receivers, but in some circumstancesa very small amount of interference energy may be received.

and produce heterodyne beat effects with the 20 transmission it is desired to receive. An object of the present invention is to provide means for suppressing such interference. I I

Owing to the high selectivity and/or low damping of the receivers with which this inven- .35- tion is concerned the percentage of modulation of the received signals is very much reduced both in the case of the signals to which the receiver may be'tun'ed and also'of interfering signals not in resonance with the receiver. The interfering volume owing to the high selectivity and since the percentage modulation is decreased, little or no modulation efiects of the interference are produced. The interference can, therefore, for all ordinary purposes be obviated by suppressing the basic frequency of the interference or the heterodyne beat note it produces with the basic frequency of the desired signals.

- The invention therefore comprises a wireless or like receiver in which there is provided a highly selective and/or low damped resonator such that an interfering modulated transmission is reduced therein to a substantially non-modulated form, in combination with means (e. g. one or more vi=5: filters or the like) for suppressing this inter transmissions located closer together than is per-.

signals are, however, received at a very small heterodyne effect comprises afilter (e. g. a mechanical resonator) sharply tuned to the bet erodyne beat note. and by-passing the electrical I effects thereof. By this means a heterodyne note which is-within therange of modulation signals being, received may be suppressed. In another form for suppressing heterodyne beat notes higher in frequency than the highest modulation signal required for reproduction,. there is provideda low-passfiltrfor the modulation signal frequencies. Also instead of orin 5 addition to the aforesaid low-pass filter for modulation signal frequencies there may be pro vided a band-pass filter for stopping all frequencies difie'ring fromthe carrier or basic frequency of the signals (ora conversion of this YQ frequency) by an amount equal .to thefrequency of the highest modulation signal required for re- .productiomthis band-pass filter having a sharp cut-01f for the'band and comprising, for ex- 1 ample, a high-pass filter and a low-pass filter. Several examples of receiversaccording to the inventionare hereinafter described with 'ref- I erenceito the accompanying drawing indicating such receivers infa purely'diagrammatic manner Figure 1 is a general diagram of'ahighly selective receiver employing a sharply tuned filter for suppressingfa Iheterodyne: beat note; I I

Figure 2 indicates a receiver employing a lowpass filter forn' odulation signals; p i

Figure [3a receiveremploying high and lowpass filters-and; I I I I Figure 4 indicates a receiver having a sharply tuned filter for suppressing a specific interference,

together withla. low-pass filter formodulation Isignalsfi I a receiver such as. is described in my prior patents hereinbefo're identified. The receiver comprises in addition to the normal tuning'fand amplifying I devices a highly selective device such as a piezoelectric crystal, a tuning fork resonatoror very low damped tuned circuits such that when ar-' ranged for theire'ception of a given transmission aninterferingtransmissioncloseto the desired transmission in regard to the basic frequencies (for example, 2 kilocycles apart) produces only a very. small and substantially unmodulated effect as hereinbefore described. 1 The heterodyne beat 5 note prcducedwill thus be presentat the output terminals 11 of the receiving apparatus 10 and.

' in the example indicated this note will have the frequency of 2000cycles. I

According theinvention-there is provided a sharply tuned filter for icy-passing this heat note and in the example shown there is provided a tuning fork 12 and co-operating electro-magnets 13 connected respectively in series between the terminals 11 and final output terminals 14 of the receiver.

The arrangement is similar to that shown and described in my prior patent application Serial No. 514,599, filed February 9, 1931. It will be understood that one of the coils 13, when fed with energy at a frequency corresponding to the mechanical resonant frequency of the tuning fork, serves to set the forkin vibration at that frequency, and the motion of the fork produces electrical variations in the other of the coils 13 at the frequency in question.

It will thus be observed that energy at the resonant frequency of the tuningfork is transmitted from one to the other of the coils 13 and thus the tuning fork provides a low impedance path for the electrical frequency corresponding to its own mechanical resonant frequency but is not appreciably affected by other electrical frequencies. This tuning fork filter is arranged in parallel with the output terminals 1 10f the apparatus, and since the tuning fork is arranged to have a resonant frequency thatis the same as the frequency of the heterodyne note it is desired to eliminate the corresponding electrical effects are by-passed by the tuning forlc and do not appear in the loud-speaker orother instrument which i may be connected to the output terminals 14. If

desired two or more heterodyne eifects may be eliminated in like manner by using corresponding tuning forks and moreover sharply tuned devices other than tuning forks may be employed if desired. Also, mechanically operative means, or an acoustic filter may be employed to prevent the reproduction of the heterodyne note in a signal reproducing device such as a loudspeaker.

According to present conditions it is usually accepted that for telephony and. music signal frequencies above about 5000 cycles are not neces sary for. satisfactory reproduction. I

In these circumstances it is possible to suppress .all heterodyne and other interferences at audio frequencies higher than 5009 cycles or whatever other limit is required by employing instead of a sharply tuned filter to eliminate only one audio frequency, as described with reference to Figure 1, a filter which has a sharp cut-off in the neighborhood ,of .5000 cycles for suppressing all higher frequencies. In Figure 2 there is indicated at 10 areceiving apparatus which is highly selective', as he ibefore described, such that in separating stations closely adjacent as regards ire-- quencies, the interfering transmission is reduced -to substantially unznodulated form. Thus, for

example, an interfering transmissicn spaced from the wanted transmission by6000 cycles and having side-bands which overlap the side-bands of thew'anted signa 5 produces substantially no interference with the exception of the beat-note between the carrier waves of the interfering and wanted transmissions. To the output of the highly selective receiver 10 there'is connected 2. lowpass filter indicated at 15. This low-pass filter, which may be of usualconstruction comprising series connected'inductances and shunt connected condensers, is designed to pass the signal frequencies below, say,,5,000 cycles and stop all heterodyne and other signal effects of higher frequencies. Thus, by means'of this low-pass filter the heterodyne beat-note of 6000 cycles as given in the example referred to above is suppressed.

The combination of the very highly selective receiver and the low-pass filter for audio frequencies consequently provides reception with substantial freedom from interference of unwanted transmissions removed from the carrier wave of the Wanted transmission by more than 5000 cycles or other limit which may be required in any particular case.

It will be understood that instead of employing a low-pass ter for audio signal frequencies as described when reference to Figure 2 or,'if desired, in addition thereto, a band-passfilter may be employed for the high frequency signals that are applied to the receiver or to frequency conversions of these signals. Thus, as indicated in Figure 3, the received signals are first applied to a high-pass lter l6 and then through a lowfilter 17 arranged to pass signal energy only of those frequencies within a band extending, for example, 5000 cycles above and below the basic or carrier frequency of the wanted transrr sic-n. The output from the filter 17 is applied to highly selective receiving apparatus indicated at 10. It will be appreciated that this arrangement of high and low-pass filters is very in effect to the arrangement described with reference to Figure 2 employing an audio r -uency filter having a sharp cut-off in the neighborcd of 5000 cycles. Thus, by means of the high and low-pass filters no signal energy of frequencies more than 5000 cycles from the frequency of the carrier wave of the wanted trans mission is permitted to be passed on in the receiver with the consequence that no heterodyne beat eifc-ets (involving the wanted carrier) of frequencies higher than 5000, cycles are produced.

As above indicatedinsteadj of employing a filter such as described with reference to Figure 1 for suppressing the heterodyne beat note produced by the carrier wave of an unwanted transmission heating with the carrier 'wave'of the wanted transmission, the carrier wave of the unwanted transmission may itself besuppressed by means of the filter which is sharply tuned to this frequency. Such an arrangement is incorporated in the receiving apparatus diagrammatically shown in Figure In this apparatus, which is of the superheterodyne type, there is shown a detector valve 20 which is connected, to a frame aerial 21 and to a local oscillator 22 in known manner. The detector valve is connected to an intermediate constant-frequency amplifier generally indicated at 23 comprising in this instance a plurality of tuned circuits coupled in cascade. The output of the intermediate frequency amer is connected to a second detector valve 24 and the resulting audio frequency signals are passed through a correcting device 25 to the output valve-26. The intermediate frequency-amplifer 23 provides a very high degree of selectivity with he result that the audio-frequency signals are distorted, the higher notes being'diminished with respect to the low notes. This distortion is conpensated for by the correcting device 25 which has a rising amplitude frequency characteristic for this purpose. This receiver thus far described is of the kind concerned in my prior patents hereinbefore identified. With this receiver the interference produced by an unwanted transmission closely adjacent as regards frequency to the wanted transmission to be received is reduced as beat-note between the carrier waves of the unwanted and wanted transmissions.

Assuming that it is required to receive all modulation frequencies up to 5000 cycles, which is according to present practice, and the carrier wave of the interfering transmission is spaced from the carrier wave of the wanted transmission by less than 5000 cycles, there is inserted in the intermediate frequency amplifier 23 a highly selective filter to suppress the carrier wave of the unwanted transmission. As shown in the final tuned circuit of the intermediate frequency circuit amplifier 23, there is provided a piezo electric crystal 2'? having a resonant frequency equal to the interfering carrier wave as converted in this instance by the oscillator 22. By suppressing this interfering carrier wave, therefore, no heterodyne beat-note is produced, and consequently no interference appears in the final output of the receiver.

In order to prevent interference by heterodyne beat-notes resulting from the wanted carrier wave heating with interfering carrier waves more than 5000 cycles removed, there is provided a low-pass filter indicated at 28 which is designed to pass all audio signal frequencies below, say,

5000 cycles and suppress all heterodyne and other audio effects of higher frequencies, as more specifically described above with reference to Fig ure 2. 7

Thus, in the arrangement shown in Figure 4 the heterodyne beat-notes produced by combination of the wanted and unwanted carrier waves and appearing'either in the required audio frequency range or outside of that range are suppressed, thus providing highly disturbance-free reception. r

Owing to the'very high selectivity of thereceiving apparatus distortion may be introduced and means may be employed, for example, as described in my prior Patents Nos. 1,821,032 and 1,821,033 for the correction of this distortion; such correction means may be included in the receiving apparatus indicated at 10.

modulated by a range of wanted signal ,fre-' quencies, the cornbinationof a resonantdevice having such high selectivity that interfering modulated carrier wave signals, whereof the carest modulation frequency of the wanted signals,

- are reduced to substantially non-modulated form,

the remaining carrier wave of the interfering signals heterodyning with the carrier wave of the wanted signals and an output'circuit for said resonant device including a modulation frequency filter for' suppressing the resulting heterodyne beat-note. V

3. In apparatus for receiving a carrier wave modulated by a range of wanted signal frequencies, the combination of a resonant device having such high selectivity that interfering modulated carrier wave signals, whereof the carrier wave differs from the carrierwave of the wanted signals by an amount less than the highest modulation frequency of the wanted signals, are reduced to substantially non-modulated form, the remaining carrier wave of the interfering signals heterodyning with the carrier wave of the wanted signals, and an output circuitfor said resonant device including a sharply tuned mechanically resonant filter for modulation signal frequencies to suppress the resulting heterodyne beat-note.

modulated carrier wave signals, whereof thecarrier Wave differs from the carrier wave of the wanted signals by an amount less than the highest modulation frequency of the wanted signals, are reduced to substantially non-modulated form, an output circuit for said resonant device including a filter for suppressing the remaining carrier wave of said interference and a modulation frequency filter for suppressing'modulation signal frequencies of a' higher order than the highest modulation signal required for reproduction. I V

5. In apparatus for receiving a carrier wave modulated by a range of wanted signal frequencies, the combination of a resonant device having such high selectivity that interfering modulated carrier wave signals whereof the car.- rier wave differs from the carrier wave of the Wanted signals by an amount less than the highest modulation frequency of the wanted signals, are reduced to substantially non-modulated form, a filter for suppressing the interference of the remaining substantially non-modulated carrier 'wave, and an additional filter for suppressing modulation signal frequencies of a higher order than the highest modulation signals required for reproduction. p JAMES ROBINSON. 

